The Chargers running back suffered a broken clavicle on the third offensive play of Thursday night’s 21-13 preseason win against the Packers. He is expected to miss four to six weeks and will have surgery today, coach Norv Turner said.

The exhibition had its highs, including rookie outside linebacker Melvin Ingram’s impressive debut, but the start was the clear low.

Mathews’ injury could linger a couple weeks into the season, a test for a roster that had its depth bolstered in the offseason.

Turner called it “part of the game.”

“That’s why Ronnie Brown’s here,” Turner said. “I think you’re going to go through situations where you don’t have everybody. That’s what I’ve been talking about since April after the draft and after we signed these players, we’ve protected ourselves in those situations.

“If someone goes down, we have answers. We have guys who can step in and go.”

Mathews is entering his third year in the NFL.

He’s had his strongest offseason, earning praise from coaches and teammates for his work ethic and strides in his game, both physical and mental.

Some players learned of the injury’s severity with the locker room thinning.

“It’s very unfortunate,” cornerback Antoine Cason said. “It sucks. This is the first time I’m hearing about it. All I can say is it’s unfortunate for a guy with so much potential. He’s been working his butt off. That’s all he can ask of himself, to hold himself accountable for us and for himself.”

Mathews, a first-round pick in 2010, had 222 carries, 1,091 yards and six touchdowns in 14 games last year. He also caught 50 passes for 455 more yards.

“It’s just unfortunate,” quarterback Philip Rivers said. “His first carry of the season. He’ll stay strong. We’ll keep him strong and upbeat because I know how excited he was about the season. He was openly commenting about how excited he was and how hard he’s trained. He’ll still bounce back and have a great year.

“As soon as he’s healthy and ready, he’ll get back out there.”

Thursday was Ingram’s NFL debut.

His arms stretch out to 31½ inches.

That was too short to impress NFL draft experts in April but long enough to beat an offensive tackle, force an MVP quarterback into an interception, and prompt double teams at Qualcomm Stadium.

Like rookie minicamp, like OTAs, like minicamp, like training camp, the No. 18 pick was sharp, starting in place of outside linebacker Jarret Johnson (hamstring) and playing into the first half.

Ingram’s forced interception of Aaron Rodgers — Cason caught the wobbler — was one of five turnovers in the first half.

“He brings a physical presence,” Turner said of Ingram. “He’s brought it from the first day he got here. He showed up at a high level when we put pads on. He’s learning, so I think he’s more comfortable with what we’re doing. He’s going to even be more physical and more explosive.”

Said safety Eric Weddle: “He’s as advertised.”

Rivers had 14 snaps.

The final two ended in turnovers.

In the red zone, Rivers forced a short pass to a tightly covered Robert Meachem. Packers cornerback Tramon Williams had position and made the interception, returning it 13 yards.

“He’s a good DB that likes to guess and get a little nosy,” Rivers said. “He hunkered down there and just drove on the ball. Obviously, I gotta see that and not throw it. Better now than later.”

After a Packers three-and-out, Rivers handed off to undrafted rookie running back Michael Hayes, who fumbled shortly after the exchange.

Two plays. Two fumbles. Rivers trotted onto the field on the Chargers’ next drive, but he was called back to the sideline and replaced by rookie Jarrett Lee.

“We just wanted to get in more of a rhythm,” Rivers said.

“Honestly, you love short fields, which the defense got us twice. In this kind of game when you’re not going to play very long, it would have been nice to drive the ball 80 yards and get into a little flow.”

Rivers finished 3 of 5 for 37 yards, including a 23-yard touchdown pass over the middle to tight end Antonio Gates.